Hi Felix,
This is a very good question. If this is from an actual exam question, I would like to know more about it as well.
I couldn't find any material from the books so I will speak from actual experience. From my previous projects, resource optimization/schedule compression are usually a response to an approved change request. (Ex. Customer wants more feature -> change request -> scope increase = I asked the team if they can stay later at work to complete the additional work packages)
Other mods please feel free to chime in.